U.S. patent application number 12/437358 was filed with the patent office on 2009-11-12 for method and system for adaptive orthogonal frequency division multiplexing using precoded cyclic prefix.
Invention is credited to XIANBIN WANG.
Application Number | 20090279626 12/437358 |
Document ID | / |
Family ID | 41266855 |
Filed Date | 2009-11-12 |
United States Patent
Application |
20090279626 |
Kind Code |
A1 |
WANG; XIANBIN |
November 12, 2009 |
Method and System for Adaptive Orthogonal Frequency Division
Multiplexing Using Precoded Cyclic Prefix
Abstract
A method for adaptive signal communication on a wireless or
wireline network is disclosed including detecting the communication
environment or determining the communication requirements, for
communication on the wireless or wireline network. The method may
include determining system parameter information for adaptive
Orthogonal Frequency Division Multiplexing (OFDM) based on the
communication environment or communication requirements and
encoding the system parameter information into at least one
precoded cyclic prefix (PCP) sequence. The method further provides
for generating an OFDM symbol transmission by combining at least
one PCP, and an adaptive OFDM symbol, using the system parameters,
then transmitting the signal from at least one OFDM transmitter to
at least one OFDM receiver followed by demodulating the at least
one PCP, and demodulate the OFDM signal using the system parameters
recovered. A related OFDM system for implementing the method for a
wireless or wireline network or platform is disclosed as are
wireless or wireline devices operable with this method.
Inventors: |
WANG; XIANBIN; (London,
CA) |
Correspondence
Address: |
MILLER THOMPSON, LLP
Scotia Plaza, 40 King Street West, Suite 5800
TORONTO
ON
M5H 3S1
CA
|
Family ID: |
41266855 |
Appl. No.: |
12/437358 |
Filed: |
May 7, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61051220 |
May 7, 2008 |
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Current U.S.
Class: |
375/260 |
Current CPC
Class: |
Y02D 30/00 20180101;
H04L 27/2607 20130101; H04L 27/2605 20130101; H04L 1/0041 20130101;
H04L 1/0025 20130101; Y02D 30/30 20180101 |
Class at
Publication: |
375/260 |
International
Class: |
H04L 27/28 20060101
H04L027/28 |
Claims
1. A method for adaptive communication signal communication on a
wireless or wireline network comprising the following steps for
transmission of an adaptive communication signal: a. detecting the
communication environment or determining communication
requirements, for communication on the wireless or wireline
network; b. determining system parameter information for adaptive
OFDM based on the communication environment or communication
requirements; c. encoding the system parameter information into at
least one precoded cyclic prefix (PCP) sequence; d. generating a
OFDM symbol transmission by combining at least one PCP, and an
adaptive Orthogonal Frequency Division Multiplexing (OFDM) symbol,
using the system parameters encoded in the corresponding PCP; e.
transmitting the signal from at least one OFDM transmitter to at
least one OFDM receiver; f. demodulating the at least one PCP; and
g. demodulating the OFDM signal using the system parameters
recovered in step (fi.
2. The method of claim 1 wherein the system parameter information
encoded to the PCP is applied by operation of an Orthogonal
Frequency Division Multiplexing (OFDM) wireless or wireline
communication platform.
3. The method of claim 2 further comprising the step of the OFDM
transmitter and the OFDM receiver adapting a communication link
therebetween using the PCP.
4. The method of claim 3 wherein the OFDM transmitter includes or
is linked to a spectrum sensing and controlling unit (SSCU), and at
least one OFDM signal generator is operable to generate one or more
OFDM symbols, comprising the further step of the OFDM signal
generator generating one or more OFDM symbols using variable system
parameters including a data carrying multicarrier modulated signal
section and the PCP.
5. The method of claim 4 wherein the data carrying multicarrier
modulated signal is generated using the inverse Discrete Fourier
Transform (IDFT), with its size controlled by the SSCU.
6. The method claimed in claim 4 wherein the PCP comprises at least
one signal sequence, representing the OFDM wireless or wireline
communication platform, and controlling information sent to the
OFDM receiver.
7. The method of claim 6 wherein the PCP for each OFDM symbol is
changed from one OFDM symbol to another, depending on the
controlling information from the SSCU.
8. The method of claim 6 wherein the PCP is combined with one OFDM
symbol generated by the system parameter carried by the PCP
depending on the controlling information from the SSCU, and the
data information to be transmitted wherein the transmitter includes
one additional PCP and a guard time which is no less than one PCP
duration before new PCP and OFDM symbol with a new system parameter
can be used and the transmitted signal can adapt its bandwidth by
changing the size of the inverse Fourier transform.
9. The method of claim 5 wherein the at least one PCP signal
sequence is a complex Kasami sequence.
10. The method of claim 4 comprising the further steps of: a.
generating an identification element of the sequence which
represents identification of the transmitter and remains unchanged;
and b. a signal parameter element is modulated by the controlling
information from SSCU.
11. The method of claim 4 wherein the at least one signal sequence
is precoded by transmitter information and system parameters,
including one or more of modulation and coding schemes,
transmission bandwidth, and carrier frequency.
12. The method of claim 1 comprising the additional step of
determining the controlling information from parameters of the
multimedia communication data stream from an associated binary
information source.
13. The method of claim 1 comprising the further step of
identifying an OFDM transmitter by differentiating received signals
by their transmitting source using the identification element of
the PCP sequence.
14. The method of wherein the method comprises the additional step
of a frame synchronization method using the correlation between the
received signal and the identification element of the PCP
sequence.
15. The method of claim 1 comprising adaptive communication signal
receiving, such adaptive communication signal receiving including
the further step of canceling or reducing interference so as to
remove or reduce intra-carrier interference (ICI) or inter-block
interference (IBI) caused by the introduction of the PCP, using the
identified PCP and the estimated channel impulse response.
16. The method claimed in claim 10 wherein the method comprises the
additional step of a spectrum sensing technique for unsynchronized
PCP-OFDM and conventional OFDM signals which are based on a. the
correlation of between the spectrum from the identification element
of PCP sequence and received PCP-OFDM signal, wherein the spectrum
OFDM signal is computed from the signal segment with duration of
N+Ncp samples, where N and Ncp are the duration of the OFDM symbol
and cyclic prefix, and the spectrum of the PCP sequence are
computed from zero-padded signal data part with duration of N+Ncp
samples, where all following N samples are set to zero; and b. the
correlation of between the spectrum from the local in-band pilots
and received conventional OFDM signal, wherein the spectrum OFDM
signal is computed from the signal segment with duration of N+Ncp
samples, where N and Ncp are the duration of the OFDM symbol and
cyclic prefix, and the spectrum of the pilot are computed from
pilot signal with duration of N+Ncp samples, where all the data
carrying subcarriers are set to zero.
17. A method for adaptive communication signal communication on a
wireless or wireline network comprising the following steps for
transmission of an adaptive communication signal: a. generating an
Orthogonal Frequency Division Multiplexing (OFDM) transmission by
combining at least one precoded cyclic prefix (PCP) and an adaptive
OFDM symbol using system parameters encoded in the corresponding
PCP; b. transmitting the signal from at least one OFDM transmitter
to at least one OFDM receiver; c. demodulating the at least one
PCP; and d. demodulating the OFDM signal using the system
parameters recovered from step (c).
18. An adaptive Orthogonal Frequency Division Multiplexing (OFDM)
system for providing a wireless or wireline network or a wireless
or wireline communication platform, the OFDM system comprising or
being linked to at least one OFMD receiver and at least one OFDM
transmitter, wherein the OFDM receiver and OFDM transmitter are
operable to adapt their communication link based on variable
transmission parameters using a precoded cyclic prefix (PCP).
19. The system of claim 18, PCP can represent one or more of the
following transmission parameters: spectrum sensing, sharing and
bandwidth control, location information, transmission parameters,
transmission power control information, or other receiver and
transmitter interaction information.
20. A wireless or wireline device comprising an OFDM transmitter
and optionally an OFDM receiver, the wireless or wireline device
being connectable to a wireless network for wireless or wireline
communications, wherein the OFDM transmitter and the OFDM receiver
are operable to facilitate adaptation of wireless or wireline
communications to address variable transmission parameters in the
wireless or wireline network, using a precoded cyclic prefix (PCP)
in the wireless communications.
21. A wireless or wireline communication management server
comprising one or more network servers connectable to a wireless or
wireline network, wherein a plurality of wireless or wireline
devices are connectable to the wireless or wireline network, and
the one or more network servers are operable to manage wireless
communications between the wireless devices on the wireless or
wireline network, wherein the one or more network servers include
or are linked to a computer program operable to enable the one or
more network servers to: a. enable processing of one or more
wireless or wireline transmissions that include at least on
precoded cyclic prefix (PCP) using an adaptive Orthogonal Frequency
Division Multiplexing (OFDM) system, said PCP being based on
variable signal parameters associated with the wireless or wireline
device sending the wireless transmission; and b. adapting one or
more wireless network parameters based on the variable signal
parameters based on the wireless or wireline transmissions
including the PCP.
22. A wireless or wireline device computer program comprising
computer instructions which when made available to a wireless or
wireline device connectable to a wireless or wireline network are
operable on the wireless or wireline device to enable (1) the
generation of a wireless or wireline transmission comprising at
least one PCP sequence based on variable signal: parameters
associated with the wireless or wireline device, (2) demodulation
of the PCP.
Description
FIELD OF INVENTION
[0001] This invention relates in general to the field of wireless
or wireline information infrastructure and more particularly to
systems and methods for adaptive wireless or wireline networks and
network devices.
BACKGROUND
[0002] Convergence of different wireless communication systems and
networks is becoming more prevalent, as well as seamless
connections between wireless and backbone wired networks. Adaptive
technologies in mobile transceiver design, network and application
services can provide an important role in supporting such diverse
mobile multimedia services. These trends in wireless communications
bring several fundamental challenges for wireless system and
network designs.
[0003] The nature of mobile multimedia communication is dynamic,
due partly to the fast variation of wireless channels, and partly
to the wide range of user applications and requirements. The user
mobility and the short wavelength of a broadband wireless signal
mean that the system throughput can vary substantially within a few
microseconds or a few feet in distance. Similarly, the traffic of
wireless communications also changes from the constant low rate
voice communications, to high sporadic internet browsing and
broadband video communications.
[0004] The traditional design methodology for mobile multimedia
communication is to devise the wireless system for the maximum data
request under the "worst case" wireless channel condition. Such a
design could result in a scenario that all the system resources are
committed to one user and no one else could be accommodated. In
contrast to the "worst-case" design methodology, considerable
bandwidth, battery power, latency, and other communication
resources can be conserved by adapting the transmission parameters
to current channel conditions and application requirements.
[0005] There is a need to develop flexible transmission
technologies which can adapt to current mobile multimedia
communication conditions and requirements in the most efficient and
reliable way.
[0006] The fast evolution of wireless communications also brings
challenge of efficient spectrum utilization. Today's wireless
communication systems are characterized by a fixed spectrum
allocation policy, i.e. the spectrum is regulated by governmental
agencies and is assigned to license holders on a long term basis
for large geographical regions. With the existing radio spectrum
regulatory framework, access to radio spectrum is frustratingly
difficult. According to Federal Communications Commission (FCC),
temporal and geographical utilization rate of the assigned spectrum
can be as low as 15% [1, 2] at any location and at any given time.
Although the fixed spectrum assignment policy generally served well
in the past, the dramatic increase in wireless communications in
recent years poses a looming challenge due to spectrum
overcrowding. Improving the spectrum utilization efficiency is
required to support the wireless communications that will continue
to fuel the economic growth. The limited availability of spectrum
and the inefficiency of its usage necessitate a new communication
paradigm termed cognitive radio to exploit the existing wireless
spectrum opportunistically.
[0007] US Patent Application, Publication Number US 2008/0014880
A1, invented by Hyon et al., discloses a signaling method between a
cognitive radio (CR) base station and a CR terminal in a CR
environment, in which a channel division method is used for the
signaling method, the method including: detecting a channel usage
of an incumbent system, which communicates with a CR base station;
sensing an outband channel to communicate with the CR base station;
receiving an EOS, which is broadcasted from the CR base station via
the outband channel according to a pre-determined period; and
transmitting a sensing report signal with respect to the channel to
the CR base station. This technique is designed for point to
multipoint communications were a base station and mobile CR users
have pre-arranged signal form a to exchange information. This is
achieved through signaling transmission using outband channel which
would require extra bandwidth.
[0008] US Patent Application, Publication Number US 2008/0080604
A1, inventor Hur et al., discloses spectrum-sensing algorithms and
methods for use in cognitive radios and other applications. The
spectrum-sensing algorithms and methods may include receiving an
input spectrum having a plurality of channels, performing a coarse
scan of the plurality of channels of the input spectrum to
determine one or more occupied candidate channels and vacant
candidate channels, where the coarse scan is associated with a
first resolution bandwidth and a first frequency sweep increment,
performing a fine scan of the occupied candidate channels and the
vacant candidate channels to determine actually occupied channels
and actually vacant channels, where the fine scan is associated
with a second resolution bandwidth and a second frequency sweep
increment, and storing an indication of the actually occupied
channels and the actually vacant channels. The signal detection
method disclosed is power/energy detection. The sensing decision is
based on the existence of signal power and may not be able to
distinguish signal from interference.
[0009] US Patent Application publication number US 2008/0089389 A1,
inventor Hu, relates to cognitive radio based wireless
communications of dynamic spectrum access networks, and in
particular to a method of addressing zero-delay frequency switching
for cognitive dynamic frequency hopping. The method combines
regular (periodic) channel maintenance with dynamic frequency
hopping over a cluster of vacated channels that are initially setup
such that the switching delays for channel setup and channel
availability check are eliminated. The method disclosed does not
manipulate the physical layer.
[0010] Cognitive radio is a revolutionary technology that provides
improvements in efficiency of spectrum usage. Ever since Joseph
Mitola III [3, 4] established the phrase "cognitive radio" in his
thesis, many definitions of what a true cognitive radio can look
like have been discussed in literature. The cognitive radio is
normally defined as an intelligent wireless communication system
that is aware of its environment and uses the methodology of
"understanding-by-building" to learn from the environment and adapt
to statistical variations in the input stimuli, with the efficient
utilization of the radio spectrum as the primary objective [5]. The
Federal Communications Commission (FCC) defines cognitive radios as
radio systems that continuously perform spectrum sensing,
dynamically identify unused spectrum, and then operate in this
spectrum at times when it is not used by incumbent radio systems
[1]. Modern wireless LAN IEEE 802.11 devices operate with a
listen-before-talk spectrum access and with dynamically changing
frequencies and transmission power [6, 7]. However, such existing
standards provide only a subset of the required techniques for
cognitive radio, and do not cover the full range of objectives for
efficiently using the spectrum. On the other hand, the terrestrial
TV broadcast band is currently in the process of being reorganized
for the roll-out of digital video broadcast [8, 9]. This change is
pursued in parallel in many regulatory domains worldwide. With the
introduction of the single frequency transmission network and
advanced equalization technique, the total number of the Digital TV
channels would be significantly reduced to maintain the current
terrestrial TV coverage [10]. It is therefore envisioned to allow
such unlicensed reuse of the some of the TV broadcast band for
cognitive radios that scan all TV channels throughout the band and
operate only upon identification of spectrum opportunities.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The invention will be better understood and objects of the
invention will become apparent when consideration is given to the
following detailed description thereof. Such description makes
reference to the annexed drawings wherein:
[0012] FIG. 1 illustrates in flow chart form the method of one
embodiment of the present invention.
[0013] FIG. 2(a) illustrates the transmitting of a wireless
transmission with the common interface utility according to one
embodiment of the present invention.
[0014] FIG. 2(b) illustrates the receiving of a wireless
transmission with the common interface utility according to one
embodiment of the present invention.
[0015] FIG. 3(a) illustrates a transmitter according to one
embodiment of the present invention.
[0016] FIG. 3(b) illustrates a receiver according to one embodiment
of the present invention.
[0017] FIG. 4 illustrates signal propagation of one OFDM symbol and
its neighboring PCPs according to one embodiment of the present
invention.
[0018] FIG. 5(a) illustrates a sample generated for one embodiment
of the present invention using an m-sequence.
[0019] FIG. 5(b) illustrates a sample generated for one embodiment
of the present invention using a Gold sequence.
[0020] FIG. 5(c) illustrates a sample generated for one embodiment
of the present invention using a Kasami sequence with n=6.
[0021] FIG. 6 illustrates the demodulation complexity of PCP-OFDM
and CP-OFDM systems.
[0022] FIG. 7 illustrates the probability of detection error for
one embodiment of the present invention with the duration of the
Kasami sequence used in the simulation is 63.
[0023] FIG. 8 illustrates the probability of detection error for
one embodiment of the present invention with the duration of the
PCP used in the simulation is 255.
[0024] FIG. 9 illustrates the probability of detection error for
one embodiment of the present invention with the duration of the
PCP used in the simulation is 1023.
[0025] FIG. 10 illustrates the symbol error rate for the PCP-OFDM
and conventional OFDM systems, using the number of the subcarriers,
N, precoded cyclic prefix duration, P, and the modulation scheme in
the PCP-OFDM systems are 256, 63 and 16QAM, respectively.
[0026] FIG. 11 illustrates in block diagram form one embodiment of
the OFDM spectrum sensing technique.
[0027] In the drawings, embodiments of the invention are
illustrated by way of example. It is to be expressly understood
that the description and drawings are only for the purpose of
illustration and as an aid to understanding, and are not intended
as a definition of the limits of the invention.
DETAILED DESCRIPTION
Overview
[0028] Recent development in cognitive radio (CR) and variable-rate
multimedia communications bring significant technical challenges in
the design of robust adaptive transmission technique in hostile
communication environment due to the strong interference and the
diverse data rate requirement and channel conditions. For cognitive
radio communications, reliable spectrum sharing and sensing
mechanism is also needed to ensure trustworthiness of the CR
communications.
[0029] The present invention provides an adaptive Orthogonal
Frequency Division Multiplexing (OFDM) system for providing a
wireless or wireline network or communication platform that is
adaptable to variable transmission parameters. The wireless or
wireline communication network or platform may include a receiver
and a transmitter, wherein the receiver and transmitter can adapt
their communication link using a precoded cyclic prefix (PCP). It
should be understood that the present invention is not limited to
application in CR and variable-rate multimedia applications in
particular.
[0030] In one aspect of the present invention, a wireless or
wireline communication transmitter is provided, which may include a
spectrum sensing unit and a controlling unit, operable to obtain
characteristics of the environment and to adapt the transmission
based on such characteristics. In another aspect of the present
invention, a wireless or wireline communication receiver is
provided operable to communicate with at least one wireless or
wireline communication transmitter. The receiver of the present
invention, in an example of the implementation thereof, may
include: a synchronization utility; a buffer; a channel estimation
and parameter selection unit; a fast Fourier transform unit; a
frequency domain equalizer; an intra-carrier inference estimator;
and an inter block interference estimator.
[0031] In one aspect of the invention, as explained below, the PCP
may comprise at least one sequence comprising identification
elements and signal parameter elements, the resulting PCP sequence
being made available to the wireless or wireline network or
communication platform to enable communication, or wireless or
wireline network or device performance, that address variable
transmission parameters. The PCP may represent one or more of
spectrum sensing, sharing and bandwidth control, location
information and transmission parameters. In one aspect of the
present invention the PCP may be a Kasami sequence, as further
explained below.
[0032] The present invention system provides for a flexible, robust
and efficient platform for wireless or wireline transmission
communications. In one aspect of the present invention the PCP
provides an efficient way of tracing the source of a signal for
interference control and standard compliant issues. In addition,
fairness of spectrum sharing may be improved by sensing the usage
of the available spectrum.
[0033] In another aspect of the present invention a common
interface is provided to identify the information source of the
wireless or wireline transmissions and convert it to a common
standard readable by the adaptive OFDM transmitter. The OFDM
receiver is also linked to a common interface utility that can
covert the transmission to the appropriate communication
standard.
[0034] The present invention provides for a method for adaptive
communication signal communication on a wireless or wireline
network comprising the following steps for transmission of an
adaptive communication signal: (a) generating an Orthogonal
Frequency Division Multiplexing (OFDM) transmission by combining at
least one precoded cyclic prefix (PCP) and an adaptive OFDM symbol
using system parameters encoded in the corresponding PCP; (b)
transmitting the signal from at least one OFDM transmitter to at
least one OFDM receiver; (c) demodulating the at least one PCP; and
(d) demodulating the OFDM signal using the system parameters
recovered from step (c).
[0035] The present invention further provides for a wireless or
wireline transmission method comprising the steps of (a) detecting
the communication environment or determining communication
requirements, for communication on the wireless or wireline
network; (b) determining the system parameter information for
adaptive OFDM based on the communication environment or
communication requirements; (c) encoding the system parameter
information into at least one PCP sequence; (d) generating an OFDM
symbol by combining at least one PCP sequence and an adaptive OFDM
symbol using the system parameters encoded in the corresponding
PCP; (e) transmitting the signal from at least one OFDM transmitter
to at least one OFDM receiver; (f) demodulating the at least one
PCP sequence; and (g) demodulating the OFDM signal using the system
parameters recovered in step (f).
[0036] In one aspect of the present invention, one part of
transmitting the signal includes determining the available
bandwidth and transmission parameters using the spectrum sensing
results from the controlling unit. Transmitting the signal may
include the transmitter in accordance with the present invention
identifying and differentiating the signals it is transmitting
using the identification element of the PCP.
[0037] In another aspect of the present invention the signal
transmitted would contain at least one PCP which comprises at least
one sequence containing identification elements and signal
parameter elements. With the signal parameter information sent with
the signal data there is no need to resort to a handshaking
procedure to establish a communication link.
[0038] In another aspect of the present invention the signal
transmitter would include one additional PCP and a guard time
before new PCP and OFDM symbol with new system parameter can be
used.
[0039] In a further aspect of the present invention the signal
parameter elements may be adapted to include information regarding
the priority of the transmission. As one example of implementation
of the present invention, the signal parameter elements may
provided such that they include information that enables a first
signal to be assigned priority over one or more second signals, for
example by being given bandwidth priority for transmission and
connectivity.
[0040] The present in invention also provides for a wireless or
wireline device operable to generate a transmission comprising at
least one PCP sequence and by operation of an OFDM transmitter, to
transmit the transmission and, by operation of an OFDM receiver, to
receive and transmission and demodulate the at least one PCP
sequence.
[0041] In one aspect of the present invention, a plurality of
wireless or wireline devices may be linked to one or more network
servers for managing communications in a wireless or wireline
network, the plurality of wireless or wireline devices being
connectable to the network, the one or more network servers being
operable to manage wireless or wireline communications between the
plurality of wireless or wireline devices on the network based one
or more communication rules implemented using the
transmission/receiving method of the present invention.
[0042] The present invention further provides for machine readable
application that may run on a wireless or wireline device and is
adapted to generate a transmission comprising at least one PCP
sequence and is operable to transmit the transmission as well as
receive a transmission with at least one PCP sequence and is
operable with an OFDM receiver to receive a transmission and is
adapted to demodulate the at least one PCP.
[0043] The present invention meets a number of requirements
presented by recent developments in cognitive radio and multimedia
communications, including related technical challenges in the
design robust adaptive transmission technique for these
communication technologies.
[0044] The present invention method allows for overall spectrum
efficiency to be improved due to the elimination of the preambles
and handshaking signaling required when there is any change in the
CR transmission parameters, in one implementation of the present
invention. There is a need for improvement in the spectrum
efficiency can be substantial due to the fast-varying nature of the
CR channel conditions, including the carrier frequency and
bandwidth of the available spectrum. In the present invention, the
identification element of the PCP is assigned uniquely to each CR
transceiver as identification label for the OFDM signal transmitted
from a CR. Consequently, the PCP can be used as sensing
characteristics for spectrum sharing among cognitive radios.
[0045] The present invention method also allows for the power
consumption at the transmitter side to be reduced through
receiver-transmitter interaction using PCP signaling link. The
power efficiency of the wireless transmitter can be improved with
the PCP-OFDM for heterogeneous multimedia communications due to the
dynamic communication needs.
[0046] The present invention method may also have key applications
within wireline communications, including DSL or digital cable
communications. By adapting each user's bandwidth and transmission
power, the crosstalk noise among users may be minimized.
[0047] The present invention also provides for an adaptive
Orthogonal Frequency Division Multiplexing (OFDM) system for
providing a wireless or wireline network or communication platform
that is adaptable to variable transmission parameters and comprises
a receiver and a transmitter, wherein the receiver and transmitter
can adapt their communication link using a precoded cyclic
prefix.
[0048] The present invention further provides for a wireless or
wireline device operable to generate a transmission comprising at
least one precoded cyclic prefix and by operation of an OFDM
transmitter, to transmit the transmission, demodulated the PCP, and
by operation of an OFDM receiver, to receive the transmission.
[0049] In one aspect of the present invention a plurality of
wireless or wireline devices linked to one or more network servers
for managing wireless communications in a wireless or wireline
network, the plurality of devices being connectable to the wireless
or wireline network, the one or more network servers being operable
to manage wireless or wireline communications between the plurality
of wireless or wireline devices on the wireless or wireline network
based one or more communication rules implemented using the
wireless transmission method comprising the steps of: (a)
generating a transmission comprising at least on precoded cyclic
prefix (PCP) using an adaptive Orthogonal Frequency Division
Multiplexing (OFDM) system; (b) transmitting the signal from an
OFDM transmitter to an OFDM receiver; and (c) demodulating the at
least one PCP.
[0050] The present invention provides for a machine readable
application that is operable to run on a wireless or wireline
device and is adapted to generate a transmission comprising at
least one PCP sequence and is operable to transmit the transmission
as well as receive a transmission with at least one PCP sequence
and is operable with an OFDM receiver to receive a transmission and
is adapted to demodulate the at least one PCP.
[0051] 1). Flexible and robust wireless transmission techniques.
The available communication channel for cognitive radio may be
hostile. On one hand, available spectrum for CR is often corrupted
with strong co-channel and adjacent-channel interference from
existing licensed communication systems. The present invention
provides a wireless CR transmission technique that is robust in
handling strong interferences. In the meantime, it is flexible and
efficient in achieving higher system capacity with varying channel
conditions. The present invention supports making variable
bandwidth available to higher system capacity. In addition, in
connection with fasting variation of the carrier frequency and
bandwidth for the available spectrum, the present invention enables
adjustment of transmission and receiving parameters in a fast and
efficient manner.
[0052] 2). Reliable spectrum sharing and sensing techniques. The
successful deployment of CR networks and the realization of their
benefits depend on the reliable and fair spectrum sharing
mechanism. Consider the following two scenarios. If a CR user
detects the presence of incumbent signals in the current band, it
must immediately switch to one of the fellow candidate bands. On
the other hand, if the secondary user detects the presence of an
unlicensed user, it should either switch to another available
spectrum or invoke a coexistence mechanism to share spectrum
resources. The first case depends on the trustworthiness of the
spectrum sensing of the primary user. Since the primary users'
usage of licensed spectrum bands can be sporadic, a CR preferably
monitors for the presence of incumbent signals in the current
operating band and candidate bands. The second scenario indicates a
transmitter identification signal should be introduced to the
cognitive radio for spectrum sharing and monitoring purposes.
[0053] 3). Interference control for regulation compliant issues. In
traditional wireless communication systems, algorithms for system
management, such as power control and channel selection, are
implemented in many radio devices, but may be vendor-specific and
invisible to the outside world, particularly the spectrum
regulators. As a result, today's standards and regulations may
constrain parameters like power levels and frequency ranges for
operation, to achieve a minimum level of interference to the
primary user and secondary users. The unique characteristic of
cognitive radios on the other hand is that their radio resource
management algorithms are weakly constrained by standards or
regulation. This implies that the entire decision-making in
spectrum management should be visible to the outside world, and
signals transmitted from a CR should be traceable to minimize the
interference to incumbent signals. In addition, transmission system
parameters of each CR should also be transparent to other CR users
to minimize the mutual interference and reliable transmission.
[0054] There is a need to address the aforementioned challenges
with the proposed adaptive OFDM systems by using PCP. OFDM is
envisioned as a key technology for broadband wireless
communications due to its high spectral efficiency and robustness
to multipath distortions [11-14]. There is a further need for the
proposed PCP-OFDM to provide a flexible, robust, and efficient
platform specifically tailored for cognitive radio
communications.
[0055] The precoded cyclic prefix, in one implementation of the
present invention, using two Kasami sequences precoded by the
transmitter identification and transmission system parameters,
provides in one aspect of the invention several important
functionalities for cognitive radio. Besides PCP's basic role as a
guard interval to eliminate intersymbol interference (ISI),
transmission system parameters including the total number of the
OFDM subcarriers, carrier frequency, and modulation and coding
schemes can be sent concurrently with any OFDM symbol. The present
invention enables avoidance of the tedious handshaking procedure to
establish a communication link.
[0056] Further, PCP provides an efficient way of tracing the source
of any CR signal for interference control and standard compliant
issues. Fairness of spectrum sharing could be improved by sensing
the usage of the available spectrum. In addition, Time Division
Duplexing (TDD) technique could be used in PCP-OFDM for the
partition of the uplink and downlink of the CR communications.
Channel conditions for uplink and downlink will use the same
frequency and experiences similar multipath distortions. As a
result, spectrum management and adaptation of the physical layer is
much easier. By changing the duplexing ratio of the TDD scheme,
different data rates for uplink and downlink can be supported. This
is of great importance as future communication data can take
different form with large variation in its data rate. The multiple
functionalities of the PCP make the new OFDM system ideal for the
cognitive radio communications.
I. OFDM Systems with Pseudo-Random Sequence as Cyclic Prefix
[0057] The present invention provides a solution for the challenges
associated with CR and variable rate multimedia communications with
the implementation of an adaptive Orthogonal Frequency Division
Multiplexing (OFDM) system, with a precoded cyclic prefix (PCP).
The PCP, in one aspect of the invention, as stated earlier, is
combined from two precoded Kasami sequences as its signal data and
signal parameter elements, and can be used for several specific
purposes related to cognitive radio, in one implementation of the
present invention. Besides the basic function as a guard interval
for the OFDM systems, the signal parameter element of PCP provides
an efficient way of sending the transmission system parameters of
the transmitter to the receivers. These parameters can include the
bandwidth, total number of OFDM subcarriers, modulation and coding
schemes used. Variable data rate transmission for multimedia
communications can be easily supported by the proposed PCP-OFDM
system.
[0058] The present invention provides for a method as illustrated
in FIG. 1, in one aspect of the invention and as explained above.
FIG. 1 further illustrates the method according to one embodiment
of the present invention comprising the steps of (a) detecting the
communication environment or determining communication
requirements, for communication on the wireless or wireline network
(100); (b) determining the system parameter information for
adaptive OFDM based on the communication environment or
communication requirements (101); (c) encoding the system parameter
information into at least one PCP sequence (102); (d) generating an
OFDM symbol by combining at least one PCP sequence and an adaptive
OFDM symbol using the system parameters encoded in the
corresponding PCP (103); (e) transmitting the signal from at least
one OFDM transmitter to at least one OFDM receiver (104); (f)
demodulating the at least one PCP sequence (104); and (g)
demodulating the OFDM signal using the system parameters recovered
in step (f) (106).
[0059] In one aspect of the present invention, communication
environment may include available spectrum bandwidth used for
transmission, channel conditions (channel variation, interference
strength, noise level). Communication requirement may include data
rate to be supported, transmission quality and accuracy requirement
in term of transmission symbol error rate, multiple streams
concurrent transmission, etc.
[0060] The OFDM wireless and wireline transmission method is
further illustrated in FIG. 2. FIG. 2(a) shows the wireless
transmitter side, while FIG. 2(b) illustrates the receiver side.
For multimedia communications, each information source (200) may be
a binary bit stream from one specific source, for example speech,
data and video sources. An example of the use of the method in the
present invention is in a health care application, each information
source is the digitized information from medical sensors such as
temperature and heart rate and other binary information sources
including audio and video streams for remote doctor-patient
interaction.
[0061] In one aspect of the present invention, the transmitter may
receive the incoming information from one or a plurality of
information sources (200). The common interface device (201) will
identify the transmission protocol and packing format. The
interface unit (201) may further remove the format related data
from the input and forward the incoming data to the PCP-OFDM
transmitter (205) and spectrum sensing and controlling unit (SSCU)
(203). The spectrum sensing and controlling unit (203) may also be
equipped with a receiving antenna (202). The spectrum sensing and
controlling unit (203) may decide the bandwidth and transmission
parameter, depending on the incoming data rates, as well as the
channel conditions from the sensing results. Multiple incoming data
streams may be combined into one single stream by this unit.
[0062] In another aspect of the present invention, the spectrum
sensing and controlling unit (203) will decide the available
bandwidth from spectrum sensing results, the data rate needs to be
transmitted, and input from the receiver controlling information
unit (204). This unit (204) may further decide the transmission
bandwidth and transmission information to PCP-OFDM transmitter
(205). The signaling information which the transmitter would like
to send to the receiver will be generated in this block.
[0063] The PCP-OFDM signal may be generated in the wireless or
wireline transmitter (205), using the information from the SSCU
(203). FIG. 2(a) further illustrates the PCP-OFDM signal may be
transmitted using the transmitting antenna (206).
[0064] Once the transmission has been transmitted it may be
received using the wireless or wireline transmission receiving
procedure as illustrated in FIG. 2(b). The signal from the wireless
or wireline transmitter (205) may be picked up using antenna (207).
The PCP signaling information and transmitted data may be recovered
using the wireless or wireline PCP-OFDM receiver (208), depending
on the controlling information from the controlling unit (209).
[0065] In another aspect of the present invention, the receiver
(208) may also report to the controlling unit (209) the receiving
performance of the wireless communication receiver (208). The
controlling unit (209) may decide the receiving algorithm used in
the receiver (208). The controlling unit (209) may also determine
any feedback information, including but not limited to power
control information, to the remote transmitter through the PCP
signaling link between the local transmitter and remote receiver.
The recovered data from the OFDM signal from the local receiver
(208) may be converted to certain format by the common interface
unit (210), depending the transmission protocol and applications.
For combined data stream by the transmitter (205), the common
interface unit (210) may divide the combined data stream back to
multiple forms. The recovered data streams will be sent to one or a
plurality of application sinks (211).
[0066] An aspect of the present invention provides for the common
interface (201) to also have the capability of combining different
data stream into one data stream for transmission at the
transmitter side, and separating each individual data stream at the
receiver side. The application sink (206) may be speaker, display
devices, or other mechanical devices.
[0067] The power consumption at the wireless transmitter side of
the wireless communication network or wireless communication
platform may be reduced using the present invention in two ways.
First, depending on the data rate to be transmitted, the
transmitter adjusts its transmission bandwidth on its own. The
transmission parameters will be sent to the wireless receiver
through PCP signaling. Second, the wireless receiver evaluates the
signal to noise ratio of the received signal and sends feedback
information to the transmitter through its PCP signaling link. The
wireless transmitter may then adjust its transmission power
accordingly.
[0068] FIG. 3 illustrates in block diagram form one embodiment of
the PCP-OFDM system. The wireless communication transmitter in FIG.
3(a) contains a system similar to the traditional OFDM system, but
with the added feature of the cyclic prefix is now replaced by a
precoded cyclic prefix which may comprise of at least one sequence.
The transmitter would include one additional PCP and a guard time
before new PCP and OFDM symbol with new system parameter can be
used. The transmitted signal can have variable bandwidth by
changing the size of the inverse Fourier transform, which is
controlled by the SSCU. Pseudo random sequence or zero sequence
have been used in OFDM as prefix and postfix to protect OFDM symbol
from ISI [15, 16].
[0069] As stated earlier, the PCP may be combined from two Kasami
sequences, precoded by the wireless or communication transmitter
identification and system parameters. The same PCP is used as the
cyclic prefix for all the forthcoming OFDM symbols unless there is
change in the transmission system parameters. The generation of the
pseudo random sequence and consequently the precoded cyclic prefix
may be controlled by the spectrum sensing, sharing and controlling
unit (SSCU).
[0070] In another aspect of the present invention, the
identification element of the PCP represents the transceiver
identification and signal parameter element is precoded for the
transmission of OFDM system parameters including the number of the
subcarriers and the modulation/coding schemes used. In addition,
the size of inverse fast Fourier transformation (IFFT) block, i.e.
the number of subcarriers of adaptive OFDM modulator, may also be
controlled by SSCU. The total number of the subcarriers in the OFDM
signal and its carrier frequency depends on the information of the
available spectrum from the spectrum sensing, sharing and
controlling unit. The number of the subcarriers as well as the
coding and modulation schemes may be coded into a different cyclic
prefix. Generation and detection of such a PCP is further discussed
below.
[0071] The following sets out an example of implementation of the
present invention:
[0072] Each OFDM symbol at the output of FIG. 3 (a) may be
specified by an N-point time-domain vector x obtained via an IFFT
of the complex data vector X of size N. Without loss of generality,
each OFDM symbol in time domain can be expressed in vector form
as
x=F.sub.N.sup.HX, (1)
where F.sub.N.sup.H=F.sub.N.sup.-1 is the inverse Fourier Transform
matrix with its (n, k)th entry (exp{j2.pi.nk/N}/ {square root over
(N)}). Operator ().sup.H denotes conjugate vector/matrix
transposition.
[0073] In one aspect of the present invention, before the
transmission of the OFDM symbol in (1), the generated PCP sequence
with length of P is inserted as its prefix. The duration of the
pseudo random length should be longer than or at least equal to the
channel delay spread for a complete removal of ISI during the
demodulation process. It should be noted that in the one embodiment
of the present invention the system contains the beginning of the
CR communication starts with one precoded cyclic prefix. This may
be equivalent to generating a new OFDM symbol of N+2P samples with
one pseudo random sequence as its last P samples and the other
sequence as its cyclic prefix in the first P samples. Consequently,
the cyclic structure for each PCP-OFDM symbol may be produced since
the pseudo random sequence may be used as cyclic prefix for all the
OFDM symbols. As a result, it creates a series of new OFDM symbols
of (N+P) samples with cyclic structure similar to traditional OFDM
symbols protected by cyclic prefix. Without loss of generality,
consider the following signal vector for interference analysis and
PCP-OFDM symbol demodulation
x'=[C.sub.P(0),C.sub.P(1), . . . , C.sub.P(P-1),x(0),x(1), . . . ,
x(N-1),C.sub.p(0),C.sub.P(1), . . . , C.sub.P(P-1)].sup.T. (2)
[0074] Now let N'=N+P and vector r' be the received signal vector
corresponding to the transmitted signal vector x' in (1). Unless
otherwise stated, assume an L-tap static complex channel
h=[h.sub.0, h.sub.1, . . . , h.sub.L-1].sup.T for signal
propagation and interference analysis, with the worst case L=P. The
received signal r or responding to the transmitted signal vector x'
can be expressed as
r ' = [ h 0 0 0 h 1 h 0 h L - 1 h 1 h 0 0 0 h L - 1 h 1 h 0 h L - 1
h 1 0 0 h L - 1 ] x ' + w ' , ( 3 ) ##EQU00001##
where the size of the channel matrix in (3) is
(N+3P-1).times.(N+2P), and w' is an additive white Gaussian noise
(AWGN) vector with the same size as r'. Suppose the channel impulse
response of the channel is known through channel estimation, a
straightforward way to obtain the equalized signal {tilde over
(x)}' with size of (N+P) in time domain can be formulated as
{tilde over
(x)}'=F.sub.N+L.sup.HD.sup.-1(H')F.sub.N+Lr'.sub.N+L+{tilde over
(w)}.sub.N+L.sup.FEQ, (4)
where r'.sub.N+L is the [N+P+1: N+2P] samples from the received
signal r' and H'=DFT.sub.N+L(h). D(H') is the diagonalized channel
matrix with the frequency channel response as its diagonal
elements. The desired equalized OFDM symbol {tilde over (x)} is the
first N samples of {tilde over (x)}'. The demodulation process may
be
{tilde over (X)}=DFT.sub.N({tilde over (x)})+{tilde over
(W)}.sup.FEQ. (5)
[0075] The complexity associated with the demodulation process for
the proposed PCP-OFDM using (4) and (5) is much higher than in a
traditional OFDM system. Compared to an N-point traditional OFDM
symbol demodulation process, one extra (N+P)-point IFFT and one
(N+P)-point FFT are required in (4). These addition are because the
frequency domain equalization is done on an OFDM symbol with size
of (N+P). IFFT/FFT with very large size can be used for cognitive
radio communications due to the dynamic range of available
bandwidth and other channel conditions. Consequently, the increase
in the demodulation complexity of the PCP-OFDM symbol could be
substantial. There is a need therefore to develop a wireless and
wireline communication receiver with reduced complexity that is
comparable to the traditional OFDM receiver. FIG. 3(b) illustrates
a wireless receiver structure for the PCP-OFDM system according to
one embodiment of the present invention.
[0076] In one aspect of the present invention, an interference
analysis is presented below for the development of the wireless or
wireline receivers according to one embodiment. FIG. 4 depicts a
static multipath channel and the received wireless communication
signal over one PCP-OFDM symbol and two adjacent PCP in (2). As
highlighted by the shaded region in the FIG. 4, the transmitted
signal appearing at the receiver may be spread by the multipath
channel, resulting in ISI. The interferences from the adjacent
blocks may have to be cancelled for the successful demodulation of
the symbol.
[0077] As illustrated in FIG. 4, only N samples in the observation
periods (OP) may be considered in the present embodiment of the
wireless receiver for the demodulation of the PCP-OFDM symbol. The
same OP is normally used in a conventional OFDM receiver. The exact
location of OP and the channel length may be determined using the
techniques in [17, 18] although alternative techniques are
considered. As a result, ISI from the preceding PCP sequence may
have to be estimated and cancelled. With estimated channel impulse
response, IST may be computed and subtracted from the received
signal. However, the inter carrier interference (ICI) still needs
to be canceled due to the elimination of the cyclic structure in
the OFDM signal when only N samples of the received signal are used
for the demodulation process.
[0078] For the signal analysis purpose, construct two N.times.N
matrices for the ISI and ICI analysis. The first matrix
C = [ h 0 0 0 0 0 h 1 h 0 0 0 0 0 h L - 1 h L - 2 h 0 0 0 0 h L - 1
h 1 h 0 0 0 0 0 h L - 1 h 0 ] , ( 6 ) ##EQU00002##
represents the channel seen by the OFDM symbol. The second
matrix
C T = [ 0 0 h L - 1 h L - 2 h 1 0 0 0 h L - 1 h 2 0 0 0 0 h L - 1 0
0 0 0 0 0 0 0 0 0 ] , ( 7 ) ##EQU00003##
represents the tail end of the channel's impulse response that
generates ISI in the succeeding symbol. These two matrices have the
interesting property of
C+C.sub.T=C.sub.cycl, (8)
where C.sub.cycl is the "ideal" channel matrix, i.e. the matrix
that results in a cyclic convolution between the transmitted signal
and the channel. Based on (3)-(7), received signal (N samples) for
the OFDM symbol in OP can be expressed as
r=Cx+C.sub.Tc.sub.P+w. (9)
[0079] To use the similar demodulation procedure for traditional
OFDM system, the following ideal received signal vector is
constructed:
r.sub.i=r.sub.l-C.sub.Tc.sub.P+C.sub.Tx, (10)
where
r.sub.1=[r'(P+1), . . . , r'(P+N)].sup.T. (11)
[0080] The signal structure depicted in (10) suggests that the
first step of the proposed hybrid domain receiver in demodulating x
is to remove the ISI term by subtracting the ISI C.sub.Tc.sub.P
from the preceding PCP sequence. For any reasonable channel
signal-to-noise ratio (SNR) of interest, the error from the
estimated channel is very small and hence there will be reliable
ISI cancellation. After ISI removal, the next step is to remove the
ICI term, or equivalently to perform cyclic reconstruction for the
received PCP-OFDM signal. This could be done iteratively as any
attempt of ICI removal should be based on a temporary decision for
the OFDM symbol. However, the computation complexity associated
with this iterative approach is enormous since the ICI cancellation
is in time domain and the demodulation the OFDM symbol is in
frequency domain. The conversion any signal from time to frequency
or from frequency to time domain will depend on Fourier
transformation.
[0081] An alternative approach may be an ICI cancellation approach
totally in time domain. Consider the propagation of the PCP-OFDM
symbol only shown in FIG. 4. When r.sub.1 is used for the
demodulation of the PCP-OFDM symbol, the remaining tail from the
PCP-OFDM symbol is actually the signal needed to reconstruct the
cyclic signal structure. To do this, the tail signal is derived
from the following received signal vector of N samples,
r 2 = [ r ' ( P + N + 1 ) , , r ' ( 2 P + N - 1 ) ( P - 1 ) Samples
, 0 , , 0 ] ( N - P + 1 ) Samples T ( 12 ) ##EQU00004##
[0082] If the signal component from the second PCP is subtracted
from (12), the desired ICI signal will be obtained [16]
n.sub.ICI=r.sub.2-C.sub.Hc.sub.P:N. (13)
where the (N.times.N) matrix C.sub.H is
C H = [ h 0 0 0 0 0 h 1 h 0 0 0 0 0 h L - 2 h L - 3 h 0 0 0 0 0 0 0
0 0 0 0 0 0 ] . ( 14 ) ##EQU00005##
[0083] Now the ideal signal for the demodulation of the PCP-OFDM
symbol can be derived using
r.sub.i=r.sub.1-C.sub.Tc.sub.p+r.sub.2-C.sub.Hc.sub.P:N. (15)
[0084] When the channel estimate is accurate, the ideal signal in
the above equation becomes
r.sub.i=C.sub.cyclF.sub.N.sup.HX (16)
[0085] As for the OFDM system with cyclic prefix, the circulant
matrix C.sub.cycl can be diagonalized by N.times.N (I)FFT matrices
[16]. For the demodulation purpose, applying a FFT matrix to the
above equation leads to
F.sub.Nr.sub.i=F.sub.NC.sub.cycF.sub.N.sup.HX=D.sub.N({tilde over
(H)}.sub.N)X. (17)
where D.sub.N({tilde over (H)}.sub.N) is the N.times.N diagonal
matrix with the estimated frequency domain transfer function as its
diagonal elements. As the result, the complete zero-forcing
demodulation process is
{tilde over (X)}=D.sub.N.sup.-1({tilde over
(H)}.sub.N)F.sub.Nr.sup.i. (18)
II. System Parameters Transmission Using Pseudo Random Sequences as
the Cyclic Prefix
[0086] As mentioned above, one difficulty for the future cognitive
radio as well as other wireless and wireline communication
transmissions are the frequent change of the system parameters due
to the fast variation of the spectrum availability and channel
conditions. Therefore an efficient way of providing transmission
system parameters to the desired wireless or wireline receiver can
improve the efficiency of the wireless communications systems,
platforms and networks.
[0087] A handshaking procedure similar to that used in licensed
communications may be difficult to achieve due to the unknown
spectrum and transmission conditions. It is therefore preferred
that the wireless transmission system parameters be transmitted in
a self-contained or concurrent manner with PCP-OFDM signals. In
addition, reliable identification of each signal from cognitive
radio or other wireless transmission device may also be needed for
interference monitoring by authorities. In this section one aspect
of the present invention, the use of a precoded cyclic prefix is
discussed.
[0088] In one aspect of the present invention, the complex PCP may
be combined from two independent Kasami sequences. Other sequences
are contemplated. The Kasami sequences may contain identification
elements and signal parameter elements which may be used to
transmit the system parameters of the cognitive radio as well as
the CR transmitter identification. It should be noted that
application of the present invention is not limited to CR
communications.
Selection of the PCP Sequence or Signals
[0089] A wide variety of signals and sequences can be used as PCP,
as long as they meet the previous discussed requirements. Also, one
desired property of the PCP is its orthogonality or
near-orthogonality, i.e., a very low cross-correlation function
between different PCPs. The other requirement is the number of the
available PCPs in the design.
[0090] Any signal or sequence with abovementioned properties can be
used in PCP-OFDM. However, for the simplicity of the wireless
system design, pseudo random sequences are preferred. Different
pseudo random sequences, including (but not limited to),
m-sequences, Gold and Kasami sequences are good candidates as they
can provide large family of the orthogonal sequences. A brief
introduction of the generation of Gold and Kasami sequence is given
as follows. Sample generators for these sequences are plotted in
FIG. 5.
[0091] The generator of Gold sequence and Kasami sequence are
summarized as follows:
Properties of Kasami Sequences.
[0092] One property of the Kasami sequences is the excellent
auto-correlation and cross-correlation properties [19-22]. In
addition, Kasami sequences provide a large family of orthogonal
codes that may be used to indicate various CR or other wireless
communication transceivers and system parameters [21]. The
identification element of the PCP is uniquely assigned to each
wireless communication transmitter for transmitter identification
purpose. The transmission parameters of the wireless signal,
including the number of the subcarriers of the OFDM signal,
modulation and coding schemes may be coded into the second Kasma
sequence as the signal parameter element of the PCP. This process
may be similar to code shift keying [23, 24].
[0093] Kasami sequence sets are one of the important types of
binary sequence sets because of their large set size and very low
cross-correlation. There are two classes of Kasami sequences: the
small set and the large set. The large set contains all the
sequences in the small set. Since it will be used as cyclic prefix
for PCP-OFDM, Kasami sequences have a period of P=2.sup.n-1, where
n is a nonnegative, even integer.
[0094] Let u be a binary sequence of length P, and let w be the
sequence obtained by decimating u by 2.sup.n/2+1. The small set of
Kasami sequences is defined by the following formulas, in which D
denotes the left shift operator, and .sym. denotes addition modulo
2[21],
K.sub.s(u)={u,u.sym.w,u.sym.Dw, . . . , u.sym.D.sup.2.sup.2-2w}.
(19)
[0095] Note that the small set of contains 2.sup.n/2 sequences. Let
v be the sequence formed by decimating the sequence u by
2.sup.n/2+1. For mod(n, 4)=2, the large set of Kasami sequences is
defined as follows [21]
K L ( u ) = G ( u , v ) [ i = 0 2 n / 2 - 2 { D i w .sym. G ( u , v
) } ] , ( 20 ) ##EQU00006##
where G(u, v) is the Gold sequence
G(u,v)={u,u.sym.v,u.sym.Dv, . . . , u.sym.D.sup.N-1w}. (21)
[0096] The correlation functions for the Kasami sequences take on
the values [21]
{ - t ( n ) , - s ( n ) , - 1 , s ( n ) - 2 , t ( n ) - 2 } , where
( 22 ) t ( n ) = 1 + 2 n + 2 2 , and ( 23 ) s ( n ) = 1 2 [ t ( n )
+ 1 ] . ( 24 ) ##EQU00007##
Properties of the Gold Sequence
[0097] The Gold sequences are defined using a specified pair of
sequences u and v, of period N=2.sup.n-1, called a preferred pair,
defined as: [0098] N is not divisible by 4, [0099] v=u[q], where q
is odd with q=2.sup.k+1 or q=2.sup.2k-2.sup.k+1. This indicates
that v can be obtained by sampling every q-th symbols of u.
[0100] The set G(u, v) of Gold sequences is defined by G(u, v)={U,
V, u.sym.v, u.sym.Tv, u.sym.T.sup.2v, . . . , u.sym.T.sup.N-1v}
where T represents the operator that shifts vectors cyclically to
the left by one place, and .sym. represents addition modulo 2. Note
that G(u, v) contains N+2 sequences of period N, which are
orthogonal to each other and may be used for transmitter
identification purpose. Having found a preferred pair, the actual
Gold codes can be generated using two shift registers as shown in
the FIG. 5(b). Note that at least one element of the Initial states
vectors must be nonzero in order for the block to generate a
nonzero sequence. That is, the initial state of at least one of the
registers must be nonzero.
[0101] The Gold Sequence Generator block outputs one of these
sequences according to the block's parameters.
One Embodiment of the PCP Using Complex Kasami Sequences
[0102] The complex PCP, combined from two independent Kasami
sequences as its identification element and signal parameter
element, can be used to transmit the system parameters of the
cognitive radio as well as the CR transmitter identification. One
property of the Kasami sequence is its excellent auto-correlation
and cross-correlation properties [19-22]. In addition, Kasami
sequences provide a large family of orthogonal codes that can be
used to indicate different CRC transceivers and system parameters
[21]. The identification element parts of the PCP, is uniquely
assigned to each CR transmitter for transmitter identification
purpose. The transmission parameters of the CR signal, including
the number of the subcarriers of the OFDM signal, modulation and
coding schemes are coded into the second Kasami sequence as the
signal parameter element of the PCP, which is similar to code shift
keying [23, 24].
[0103] Precoded Cyclic Prefix with Kasami Sequecnes. Two Kasami
sequences may be used to generate the precoded cyclic prefix
according to
c.sub.P=c.sub.P,r+jc.sub.P,i. (25)
Note here all elements in the pseudo random sequences in (25) take
on values +1 or -1. This is to avoid any direct current (DC)
component in the transmitted signal. As mentioned above, the
identification element of the cyclic prefix, c.sub.P,r, will be
used as the identification of cognitive radio, while the signal
parameter element c.sub.P,i will be used to transmit the system
parameters. The generation of each Kasami sequences is shown by the
sample generator in FIG. 5(c). In this figure, the boxes represent
shift register units, and .sym. represents modulor-2 adder or
exclusive-OR gate. In one aspect of the present invention, the
precoded cyclic prefix may use a large set Kasami sequence; both
the signal data and signal parameter element of the PCP has
M=2.sup.n/2+1(2.sup.n+1) different sequences.
[0104] In another aspect of the present invention, the
identification element of the cyclic prefix may be uniquely
assigned as the identification of the cognitive radio. Signals from
each cognitive radio can then be easily traced back to its sourcing
transmitter for spectrum monitoring and sharing purposes. With the
M possible sequences for the signal parameter element, it is
therefore possible to transmit log.sub.2M.apprxeq.1.5n bits for the
cognitive radio parameters. This approach is similar to coded shift
keying [References] and should be understood that it is not limited
to application in CR. The input data sequence is denoted as
d=[d.sub.0,d.sub.1, . . . , d.sub.1.5n-1] (26)
where d.sub.i.epsilon.{0, 1}. Each data sequence of system
parameters thus may be associated with one unique Kasami
sequence.
[0105] As illustrated in FIG. 5(c), the initial state of first
shift register is fixed to a nonzero sequence, and the second and
third shift registers are set to:
d.sub.1=[d.sub.0,d.sub.1, . . . , d.sub.n-1] (27)
and
d.sub.2=[d.sub.n,d.sub.n+1, . . . , d.sub.1.5n-1] (28)
[0106] For instance, it is possible to transmit nine bits of system
information using Kasami sequence when n=6. If the first two bits
of d in (26) are used to indicate the number of the subcarriers,
four different sizes can be used. Similarly, information of four
modulation and four coding schemes can be transmitted using the
d.sub.2 d.sub.3 and d.sub.4 d.sub.5. The remaining bits may be used
for error coding or indicating the order of the OFDM symbol when
the system information should be transmitted over several different
OFDM symbols.
[0107] An alternative way of using the system parameter information
may be to let each different d represents a different
pre-determined wireless transmission platform; with each platform
having its own combination of OFDM subcarriers, bandwidth, coding
schemes etc. As an example, the large Kasami set with n=6 provides
512 different transmission options for CR and other wireless or
wireline communications.
[0108] Synchronization, Transmitter Identification, and
Demodualtion of the PCP. The transmitter's identity, i.e. the
identification element of the watermark, will keep the mobile
receiver synchronized all the time. This may be achieved through M
correlating operations. Each of the correlation for the received
signal begin from the kth sample is
C m ( k ) = l = 0 P - 1 c P , r , m ( l ) r * ( k + l ) , m = 0 , 1
, , M - 1 ( 29 ) ##EQU00008##
where C.sub.P,r,m is a local generated Kasami sequence and m is the
transmitter identification associated with it.
[0109] In one aspect of the present invention, the above
correlation in (29) may be computed over an observation period
longer than one PCP-OFDM symbol, with M correlations for each
sample. The computation complexity associated with this process can
be as high as ANP complex multiplications. However, the cyclic
nature of the PCP-OFDM signal (with period of N and PCP length of
length P) provides a straightforward way to achieve time
synchronization, as the correlation function
C r ( k ) = l = 0 P - 1 r ( k + l ) r * ( k + l + N ) , ( 30 )
##EQU00009##
[0110] has a triangular shape with its maximum at k=k.sub.0. The
total number of complex multiplications in (30) is dramatically
reduced to PN. When the symbol duration (FFT size) of the OFDM
system is not known, a few trials with all possible values for N
may be needed for (30).
[0111] Once the timing synchronization is achieved at k=k.sub.0,
equation (29) may be used for the wireless transmitter
identification by computation the correlation function at
k.sub.0
C m , r ( k 0 ) = l = 0 P - 1 c P , r , m ( l ) r * ( k 0 + l ) , m
= 0 , 1 , , M - 1. ( 31 ) ##EQU00010##
[0112] The local Kasami sequence C.sub.P,r,m pseudo random sequence
that leads to the maximum output in (31) is the identification
sequence of the transmitting wireless transmitter. In addition, the
system parameter transmitted using the signal parameter element can
be easily demodulated by cross-correlating the received signal with
the locally generated Kasami sequence, specifically
C m , i ( k 0 ) = l = 0 P - 1 j c P , i , m ( l ) r * ( k 0 + l ) ,
m = 0 , 1 , , M - 1. ( 32 ) ##EQU00011##
[0113] The correlation in the above equation may be computed for
every sequence in the Kasami code set. The local Kasami sequence
with the largest output in (32) is the sequences coded from the
system parameters. With the one to one mapping between the
transmission parameter and C.sub.P,r,m, the original data d in (26)
used to encode the Kasami sequence may be retrieved. The overall
system parameter detection error rate is derived in the Appendix
as
P e = 1 - [ 1 - P e , m ] M - 1 . where ( 33 ) P e , m = Q ( [ A -
t ( n ) + 2 ] / 2 .sigma. n ) + 1 5 i = 1 5 Q ( [ A - t ( n ) + 2 ]
/ 2 - B i .sigma. n ) , and ( 34 ) Q ( a ) = .intg. a .infin. 1 2
.pi. - y 2 2 y . ( 35 ) ##EQU00012##
[0114] A is correlation peak of the Kasami sequence and
.sigma..sub.n is the standard variance of the noise component in
(32). B.sub.i is one of the five possible values in (22).
III. Spectrum Sensing of OFDM Signal
[0115] Sensing of Orthogonal frequency division multiplexing (OFDM)
signal in low signal-to-noise ratio (SNR) may be significant for
cognitive radio and spectrum efficient communications due to the
wide applications of OFDM in many existing and evolving broadband
wireless communications. In-band pilots, multiplexed with the
data-carrying subcarriers, provide one distinct feature of OFDM
signals. For PCP-OFDM signals, the PCP provides the time domain
characteristics for PCP-OFDM signal detection.
[0116] One embodiment of the signal sensing technique for OFDM
signal is illustrated in FIG. 11 in block diagram form. The
proposed sensing technique for conventional OFDM signal sensing
techniques may be to match the received signal y[n] with a
pre-local reference (local reference block in FIG. 11) in the
frequency domain using FFT (FFT block in FIG. 11). For conventional
OFDM signal, the local reference may be the in-band pilot signal.
For PCP-OFDM signal detection the identification element of the PCP
may be used as a local reference. The received signal from the RF
front end may be digitized using A/D converter, and then re-range
as signal segment
Segmentation of the Received Signal.
[0117] In one embodiment of the present invention, the first step
of the algorithm is to segment the received baseband signal samples
with a length of Ns which may be a complete OFDM symbol duration
including the cyclic prefix. Because of the uncorrected timing
offset between the transmitter and receiver at low SNR, the
starting point of segmentation is unknown. As a result, each
segment of the received signal may contain a complete time domain
pilot sequence with duration of Ns samples but with an unknown
timing offset.
[0118] Each segment of Ns samples may contain two incomplete
adjacent OFDM symbols. However the unknown timing offset will be
the same for all segments of the received signals. Since the effect
of unknown timing offset in frequency domain is a phase rotation,
it may therefore be possible to detect the OFDM with unknown timing
offset in the frequency domain. Therefore, the impact of unknown
time synchronization error on the OFDM spectrum sensing can be
effectively mitigated under lower signal to noise ratio (SNR)
conditions using the proposed segmentation of the received
signal.
[0119] The segmented received signal may be stored in a buffer for
future processing, including average and Fast Fourier transform
(FFT). The spectrum sensing may be realized through the frequency
correlation block in FIG. 11.
[0120] In addition, a spectrum sensing threshold determination
technique based on noise parameter cancellation (NPC) method for
OFDM signals is employed; where the frequency shifted received
signals may be correlated with the local pilot reference.
[0121] Due to the characteristics of the reference signal in the
OFDM signal, including pre-assigned pilots in conventional OFDM
system and PCP sequence in the PCP-OFDM system, the statistics of
the frequency domain correlator output T(Y) in FIG. 11 for
reference sequence and data-carrying signal is completely
different, which provides a distinct feature for the OFDM signal
detection. Taking the ratio of the two kinds of noise related
frequency domain correlations as the test statistics; the proposed
detection algorithm can eliminate the restriction of prior noise
knowledge and identify the primary signals from other potential
interference sources simultaneously. In addition, the impacts of
both time and frequency offsets are mitigated with the proposed
signal segmentation and frequency domain correlation. In PCP-OFDM
system, different PCP sequence can be used as local reference in
"Local Reference" block in FIG. 11.
IV. Example in Operation
[0122] Computer simulations have been carried out to verify and
extend the analytical results of the proposed PCP-OFDM in previous
sections. The total number of the multiplications needed for the
different receiver structures in (4) and (15)-(18) are plotted in
FIG. 6. As observed from this figure, the complexity of the
proposed demodulator is reduced substantially by using the wireless
receiver as shown in FIG. 3(b) and (15)-(18). Two FFT/IFFT
operations of (N+P)-point are saved due to the different
equalization algorithms in the proposed hybrid domain equalizer.
This is because the frequency domain equalization in (4) was
performed on the size of (N+L) samples and the demodulation of the
OFDM symbol should be on the original OFDM symbol size of
N-point.
[0123] In one aspect of the present invention, the receiver
structure in (15)-(18), equalization and demodulation of the OFDM
symbol are both on N-point scale. The removal of ISI from the
previous OFDM symbol and ICI may be achieved in the time domain.
Therefore, the conversion between the frequency to time domain is
avoided. Comparing the two equalization and demodulation
approaches, the total number of the complex multiplications is
reduced from 2(N+P) log.sub.2 (N+P)+N log.sub.2 N+P(P-1)/2 to N
log.sub.2 N+P(P-1)/2. As illustrated in FIG. 6, the total number of
multiplications is reduced from around 15,000 to 6,500 for the OFDM
system with FFT/IFFT size of 512. The number of saved
multiplication increases for a PCP-OFDM with larger FFT/IFFT
size.
[0124] To evaluate the data transmission technology using the
procoded cyclic prefix, the probability of the detection error for
the system parameters for was simulated and plotted in FIGS. 7, 8
and 9 respectively. Three different Kasami sequences are considered
in the simulation for n=6, n=8 and n=10. The corresponding periods
of these sequences are 63, 255 and 1023. Note here the Kasami
sequences for n=8 belong to a small set. It is observed that good
CR system parameter detection performance is obtained at very low
signal to noise ratio, even for the PCP with the duration of 63
samples.
[0125] Without any multipath interference mitigation, the detection
error upper bound is 10.sup.-1 at the SNR of 10 dB. However, this
number can be reduced to 10.sup.-2 when the detection is performed
on the averaged PCP over two adjacent OFDM symbols. This number was
further reduced to 10.sup.-4 when this average period is extended
to four OFDM symbols. Since the operational SNR requirement for the
OFDM system is much higher than the PCP system parameter
transmission system, robust performance is expected for the
proposed transmission scheme based on PCP.
[0126] In addition, a multipath interference cancellation technique
and RAKE receiver may be used to improve the detection performance.
Similar performance curves also exist for the PCP-OFDM using Kasami
sequences of 255 and 1023 samples as its PCP. The difference is the
detection performance was significantly improved when the duration
of the cyclic prefix increased. Very low error rate is achieved
without any multipath interference cancellation as illustrated in
FIGS. 8 and 9. As seen in FIGS. 7 to 9, very robust performance can
be achieved in the proposed the data transmission scheme using
precoded cyclic prefix, even at very low SNR levels. As a result,
no error correction coding is needed for the proposed system using
Kasami sequences with period of 255 or 1023. A simple time domain
averaging of neighboring PCP sequences can significantly improve
the performance.
[0127] Numerical simulations have also been conducted to quantify
the performance of the wireless communication PCP-OFDM receiver,
particularly the hybrid domain equalizer. The demonstration OFDM
system considered has an FFT size of 256 and PCP duration of 63
samples, which is about 1/4 of the OFDM symbols duration. PCP-OFDM
symbols are generated in the simulations as per FIG. 3(a). As for
the channel model, we consider two static channel models: [0128]
Channel 1: h.sub.1=[0.2592, 0, 0, 0, 0.8639, 0, 0, 0, -0.3455i, 0,
0.1728, 0, -0.0864i, 0, 0, 0.1728].sup.T, and [0129] Channel 2:
h.sub.2=[0.9713, 0, 0, -0.0971+0.1943i, 0, 0, 0, 0.0971].sup.T.
[0130] The first channel h.sub.1 has a longer effective impulse
response and hence a smaller coherent bandwidth. It also has more
variation in the frequency response than h.sub.2. Both channels can
be considered as "bad" channels for OFDM because of their
relatively large delay spreads, with Channel 1 depicting a more
pessimistic scenario than Channel 2.
[0131] The emphasis of this investigation is to demonstrate the
workability of PCP-OFDM and its potential application in cognitive
radio and other communications. Consequently, "exact" channel
modeling and parameter selections are only secondary concerns, as a
lot depends on design issues such as: choice of frequency of
operation; symbol rate; modulation; antenna types; antenna heights;
terrain; rates of movement; and other geometrical factors (e.g.,
distances between antennas and distances to reflective
surfaces).
[0132] The PCP used in the simulation was generated by a Kasami
sequence generator shown in FIG. 5. Two Kasami sequences with
period of 63 samples are combined together to generate the complex
PCP to be inserted into OFDM signal. The first Kasami sequence is
precoded by the transmitter identification for the cognitive radio
transceiver. The signal parameter element of the PCP is modulated
by nine bits information as the initial value to the Kasami
sequence. The nine bits information provides sufficient information
on system parameters of the cognitive radio to the receiver. It can
support the choice of 512 different transmission platforms for CR
communications, depending on the CR channel conditions including
bandwidth, interference strength and mobility.
[0133] In case of using Kasami sequence with period of 1023
samples, the number of the different transmission platform
supported can be as high as 32,768. The SER curves for 16QAM
modulation were plotted in FIG. 10. As predicted, the replacement
of the traditional cyclic prefix with the proposed precoded Kasami
sequence has virtually no impact to the OFDM symbol error rate
performance. The performance difference between the PCP-OFDM system
and the traditional OFDM system with cyclic prefix is negligible.
This can be explained by equation (15). ISI from the precoded
cyclic prefix to PCP-OFDM symbol was removed and the cyclic
structure for the received signal was reconstructed. Consequently
there is no difference in using the precoded cyclic prefix and the
traditional cyclic prefix. The equivalent performances of two
different OFDM systems are shown in FIG. 10. The simulation results
also confirm the effectiveness of our ISI and ICI cancellation
techniques. Although similar performance characteristics are
observed for Channel 2, there are, however, smaller gaps between
the SER curves of the PCP-OFDM receiver and the lower bound
achieved by the AWGN channel. Once again, this stems from the fact
that Channel 1 is a more hostile channel than Channel 2.
[0134] A new adaptive OFDM system supporting fast and efficient
spectrum allocation for future cognitive radio, variable-rate
multimedia and other wireless communications is detailed above. The
flexibility of the new OFDM platform is derived from the concurrent
transmission of the system parameters of the cognitive radio (in
one example of implementation of the invention) together with the
OFDM signal. By encoding the transmission system parameters into
the precoded cyclic prefix, preamble and handshaking signaling may
be substantially simplified during the establishment or adaptation
cognitive radio communication link under varying channel
conditions.
[0135] In addition, the primary function of the precoded cyclic
prefix as the guard interval in traditional OFDM to avoid
intersymbol interference remains. The precoded cyclic prefix also
provides an identification label for any OFDM signal emitted from a
wireless communication transmitter. The corresponding wireless
communication receiver for the OFDM system was designed based on
the intersymbol interference and intercarrier interference
cancellation algorithms.
[0136] Implementation complexity for a hybrid domain equalizer in
accordance with the present inveiis dramatically reduced and is
comparable to traditional OFDM systems. The probability of the
detection error for the proposed system parameter transmission
technique using PCP as well as symbol error rate of the PCP-OFDM
were analyzed and verified through numerical simulations. With the
PCP-OFDM platform, spectrum assignment will become more flexible
and dynamic. Greater flexibility in responding to the dynamic
channel conditions as well as various communication demands will be
easily achieved.
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